Technical Field
The present disclosure relates to a sensor for detecting hydrogen ions in an aqueous solution, e.g., the pH or CO2 level and to an apparatus for detecting hydrogen ions in an aqueous solution using said sensor.
Description of the Related Art
The concentration of hydrogen ions, i.e., the pH or CO2 in a solution and more generally in a liquid product, is a very important measurement made in many research activities and industrial manufacturing processes.
For instance, the pH is typically measured in the food and beverage industry, as well as in body care products, in pharmaceuticals, in water and waste product treatment.
Many sensors exist in the art for detecting the pH value of a liquid product, either for laboratory research or industrial detection purposes.
The paper “New Technology for the Detection of pH”, in “Journal of Biochemical and Biophysical Methods”, by Miao Yuquing et al., Elsevier B. V., 2005, describes a variety of pH sensors and provides indications about the physical operation principles of such sensors.
One of these prior art sensors, which is extensively used for pH measurement, uses glass electrodes. Prior art glass electrode-based pH sensors use a potentiometric detection method and include a measuring glass electrode and a separate reference electrode in a buffer solution containing a conductive potassium chloride (KCl) gel.
These electrodes are usually accommodated in a combined electrode containing both electrodes, which is connected to an electronic meter having a signal amplifier and temperature compensation. A silver wire enclosed in the measuring electrode allows the passage of a signal indicating the difference between the pH values of the solutions inside and outside the glass membrane.
The reference electrode has a fixed and stable potential, which is independent of the solution being measured and is calibrated outside the system in a reference solution.
The most commonly used reference electrode is a silver—silver chloride electrode placed in a saturated saline or in gel.
The measuring and reference electrodes form a circuit that allows measurement of the voltage generated by the glass electrode.
While glass electrodes allow their sensors to operate and detect pH values over the entire scale of values from 0 to 14, they still suffer from the drawback that, for detection accuracy, they must be carried in a buffer solution and maintained in a wet state.
Further potential drawbacks of pH glass electrodes include physical fragility, leakage of the reference electrode solution into the sample solution to be measured, poor response in low ionic strength solutions, high background noise and low signal-to-noise ratio.
Other known types of pH sensors are those named ISFET (ion-sensitive field effect transistors) which are used in applications for which glass electrodes are inadequate.
Namely, these transistor sensors are sensitive to hydrogen ion concentration, and their sensitive part relies on an electroactive gate that restricts electric current flow as a function of hydrogen ion concentration.
Changes in ion concentration alter the current flowing through the transistor. The most commonly used materials for pH sensitive gates include silicon dioxide (SiO2) and tantalum pentaoxide (Ta2O5).
Since ISFET sensors provide quick measurements and are less temperature-dependent than glass electrodes, they afford longer maintenance and calibration intervals, and further provide the advantage of allowing miniaturization and automation.
Nevertheless, due to a lack of linearity at the ends of the pH spectrum, they have the drawback of providing a restricted pH analysis interval, from 2 to 12.
Concerning optical sensors, they use a photodetector to measure fluorescence changes in a fluorine-based indicator, as a function of the pH.
Many fluorine-based indicators or dyes are available, which change fluorescence with H3O+ concentration.
These sensors do not require calibration but have drawbacks including irregularity, photo bleaching (which means that the optical sensors must be kept in the dark to retain their effectiveness), leaching of the pH sensitive chromophore into the sample solution beyond restricted pH ranges.
Referring now to sensors that use electrodes comprising electrically conductive polymers, generally designated as CPs, those that are known in the art use platinum electrodes with a conductive polymer comprising poly(m-phenylenediamine) or generally an anyline-, thiophene- or benzene-derivative based polymer deposited thereon, particularly by electropolymerization.
While these pH sensors have the advantage that they may be formed as an entirely solid-state device, and are substantially temperature independent, those that are currently known and tested have a relatively slow response, and do not allow effective reuse for testing different products or solutions.